APOBEC3B is an antiviral enzyme that has been implicated in cancer mutagenesis. APOBEC3B is one of seven human cytidine deaminases in the APOBEC3 (A3) family. The A3 family includes APOBEC3A (A3A); APOBEC3B (A3B); APOBEC3C (A3C); APOBEC3D (A3D) APOBEC3F (A3F); APOBEC3G (A3G); APOBEC3H (A3H).